Access to safe drinking water and to water resources for agriculture is essential for African development; water use on the continent is likely to increase substantially in the next few decades. Groundwater responds more slowly to climate variability than surface water and is therefore important, for example, in the event of drought. Yet information about groundwater resources on the African continent is woefully incomplete. MacDonald et al. report quantitative maps of groundwater storage and potential groundwater yields in Africa. To do so, they collated and reviewed existing hydrogeological maps, aquifer studies, and borehole data from across the continent. The estimated overall volume of groundwater, 0.66 million km3, far exceeds that of the annual renewable freshwater resources and of lake waters. However, groundwater distribution is uneven across the continent. Most of the large groundwater stores that would be able to accommodate high-yielding boreholes are located in northern Africa and are often far away from population centers. The authors also caution that groundwater resources must be managed carefully to avoid degradation and depletion.